(i) Field of the Invention
The present invention relates to a tap device of a cable broadcasting system in which a part of transmission signal running through a transmission line leading to a terminal side from a center apparatus is branched and transmitted to a subscriber""s terminal device, particularly to a tap device of a cable broadcasting system in which it can be switched on the side of the center apparatus whether or not the transmission signal is transmitted to the terminal device.
(ii) Description of the Related Art
In a conventional cable broadcasting system such as CATV system in which a broadcasting signal such as a television signal is transmitted to a subscriber""s terminal device via one transmission line formed of a coaxial cable, and the like, the transmission line is provided with a tap device, so-called tap-off, for leading signals in order to branch the broadcasting signal from the transmission line and leading the signal into a subscriber""s house.
Moreover, in a known tap device, it can easily be switched according to a subscriber""s request or the like whether or not the broadcasting signal is distributed to the subscriber""s house. Specifically, the tap device is provided with a relay disposed in a signal path between a directional coupler for branching a part of the broadcasting signal from the transmission line and a branched output terminal for outputting the branched broadcasting signal toward the subscriber""s terminal device, in which a high-frequency broadcasting signal can be passed with a reduced loss. The relay is a so-called high-frequency relay, and a latching relay which can hold a switched state is usually used. By driving the relay in response to a command signal transmitted from the center apparatus via the transmission line, the connected/disconnected state of the signal path, in other words, the output or cut-off of the broadcasting signal to the terminal device can easily be switched on the side of the center apparatus.
When the conventional tap device is installed on the transmission line, an individual address is allocated to the device in order to distinguish the device from the other electronic apparatuses connected to the transmission line Additionally, when the center apparatus switches the output of the broadcasting signal to the branched output terminal of the tap device, a command signal including command data and the individual address of the tap device is generated This command signal is sent to the transmission line. In this case, on the side of the tap device, the command signal having the corresponding address is selected from various command signals transmitted via the transmission line, that is, the command signal from the center apparatus is extracted. By driving the relay based on a command included in the command signal, it is switched whether or not to transmit the broadcasting signal to the branched output terminal.
Therefore, according to the conventional tap device, it can easily be switched on the side of the center apparatus whether or not to transmit the broadcasting signal to the subscriber""s terminal device from the branched output terminal. Therefore, an operator does not have to go to a place where the tap device is installed for the switching operation.
On the other hand, in the conventional tap device, a latching relay has been used as a high-frequency relay for connecting/disconnecting the signal path leading to the branched output terminal from the directional coupler, so that the power consumption in the tap device is reduced and the broadcasting signal can be distributed to the subscriber""s terminal device even during a power failure.
This latching relay is also called a keep relay or a retaining relay. The position of a movable contact can be switched by energizing a relay coil. Once switched, even when the power supply to the relay coil is cut off, the position of the movable contact can be self-retained.
Specifically, in the conventional tap device, by using the latching relay to switch the connected/disconnected state of the signal path, the power amount necessary for switching the output/stop of the broadcasting signal from the branched output terminal can be minimized. Additionally, even when any power cannot be supplied to the relay coil because of the a power failure, the output/stop state of the broadcasting signal to the subscriber""s terminal device can be retained.
In the latching relay, the position of the movable contact can usually be self-retained using a permanent magnet, and the position of the movable contact is determined by the power supply to the relay coil. Therefore, immediately after the conventional tap device is installed on the transmission line, the connected/disconnected state of the signal path leading to the branched output terminal from the directional coupler is not determined. In some cases the broadcasting signal cannot be distributed to the subscribers"" terminal devices to which the broadcasting signal should be distributed.
Even when the broadcasting signal cannot be distributed to the subscribers"" terminal devices immediately after the installation of the tap device on the transmission line, by subsequently transmitting the command signal for output switching from the center apparatus, the latching relay is driven or energized in response to the command signal, and the connected/disconnected state of the signal path is determined. Therefore, this is not a problem when a work for newly installing the tap device is performed.
However, when the tap device or a part of the tap device is once detached from the transmission line for the purpose of repairing, changing, checking and the like of the tap device, and is again installed, the distribution of the broadcasting signal to the subscribers is discontinued.
When the distribution of the broadcasting signal to the subscribers is discontinued, the distribution of the broadcasting signal needs to be quickly resumed. In the conventional tap device, however, when the tap device is installed on the transmission line, the connected/disconnected state of the signal path leading to the branched output terminal from the directional coupler is not determined until the command signal for the output switching is transmitted from the center apparatus. Therefore, in some cases, although the installation of the tap device is completed, the distribution of the broadcasting signal to the subscribers"" terminal devices is discontinued for a long time.
Wherefore, an object of the present invention is to provide a tap device which can switch between outputting and stopping a broadcasting signal from a branched output terminal in response to a command signal transmitted from a center apparatus via a transmission line, so that the broadcasting signal can quickly be distributed to subscribers"" terminal devices immediately after the tap device is connected to the transmission line.
To achieve this and other objects, the present invention provides a tap device which is connected to a transmission line leading to a terminal side from a center apparatus in the same manner as in the above-described conventional tap device, and which is provided with a directional coupler for branching a part of a transmission signal, for example, a broadcasting signal running through the transmission line to a subscriber""s terminal device. The directional coupler branches a part of the transmission signal running through the transmission line, and transmits the branched transmission signal to the terminal device via a branched output terminal. A signal path for leading the transmission signal to the branched output terminal from the directional coupler is provided with a latching relay for switching a connected/disconnected state of the signal path.
In the tap device of the present invention, a receiving circuit receives a command signal indicating whether or not to output the transmission signal transmitted to the transmission line from the center apparatus via the branched output terminal, and a first controller drives the latching relay in response to the command signal received by the receiving circuit, so that the connected/disconnected state of the signal path, in other words, whether or not to output the transmission signal via the branched output terminal is controlled in accordance with the state of the command signal.
Moreover, the tap device of the present invention is provided with an operation switch for inputting an output command to output the transmission signal via the branched output terminal by manual operation. In this case, when the output command is inputted via the operation switch, a second controller drives the latching relay and performs control to place the signal path for leading the transmission signal to the branched output terminal from the directional coupler into the connected state.
Therefore, according to the tap device of the present invention, not only can it be set, in response to the command signal outputted from the center apparatus, whether or not to distribute the broadcasting signal to the subscriber""s terminal device, but also the broadcasting signal can forcibly be distributed to the subscriber""s terminal device by operating the operation switch on the site where the tap device is installed.
According to the present invention, by operating the operation switch immediately after the installation work of the tap device onto the transmission line is completed, the distribution of the broadcasting signal to the subscriber""s terminal device can quickly be started. For example, when the repairing, changing, checking, and the like of the tap device is performed, the distribution of the broadcasting signal to the subscribers can be prevented from being discontinued for a long time. Thus, the quality of the broadcasting service to the subscribers can be enhanced.
Furthermore, the tap device is further provided with a splitting circuit for splitting the transmission signal branched by the directional coupler into a plurality of signals, and outputting the split transmission signals via a plurality of branched output terminals. In this case, latching relays are disposed in a plurality of signal paths leading to the plurality of branched output terminals from the splitting circuit. The first controller drives the latching relays of the signal paths in response to the command signals transmitted from the center apparatus, and controls the connected/disconnected states of the signal paths individually. The second controller drives the latching relays of the signal paths in response to the output command via the operation switch, and performs control to place all the signal paths into the connected states.
Specifically, in the tap device, the broadcasting signal can be distributed to a plurality of subscribers"" terminal devices via the transmission line. Moreover, the latching relays are disposed on the signal paths leading to a plurality of branched output terminals from the splitting circuit. By switching the state of each latching relay in response to the command signal from the center apparatus, on the side of the center apparatus, it can be set, for each subscriber connected to the branched output terminal, whether or not to output the broadcasting signal via each branched output terminal. Furthermore, when an operator operates the operation switch on the site where the tap device is installed, the broadcasting signals can be outputted via all the branched output terminals.
Therefore, according to the tap device of the present invention, not only can it be set on the side of the center apparatus whether or not to distribute the broadcasting signal to each of a plurality of subscribers"" terminal devices, but also the distribution of the broadcasting signal to the subscribers"" terminal devices can quickly be started by operating the operation switch immediately after the installation work of the tap device onto the transmission line is completed. Also in this case, when the repairing, changing, checking, or the like of the tap device is performed, the distribution of the broadcasting signal to the subscribers can be prevented from being discontinued for a long time, and the quality of the broadcasting service to the subscribers can be enhanced.